Such an electroacoustic sensor can implement various transduction technologies. It can in particular be of the capacitive, piezoresistive, piezoelectric, electrodynamic or optical type. Generally, such an electroacoustic transducer comprises a mobile element (such as a deformable membrane, or a suspended or deformable plate, or also a flexible strip) the movement of which, caused by an acoustic wave, is transformed into an electrical variable, which is the image of the acoustic pressure, by a transduction element.
A piezoresistive electroacoustic sensor utilizes piezoresistive gauges placed in the zones of maximum stresses of a membrane constituting the mobile element.
A piezoelectric electroacoustic sensor utilizes a piezoelectric covering placed over a membrane constituting the mobile element and electrodes configured in order to characterize the stresses in the membrane.
An electrodynamic electroacoustic sensor utilizes a coil and magnets in order to perform a measurement of the current when the coil borne by the mobile element is displaced in a fixed magnetic field.
An optical electroacoustic sensor utilizes an optical measurement of the displacement of the mobile element.
The capacitive detection is that which offers the greatest sensitivity to the small displacements of the mobile element, and thus constitutes the technology that is preferentially, but not necessarily, implemented in the invention.
An electroacoustic sensor with a capacitive effect, also called an electrostatic transducer, comprises a mobile electrode positioned opposite a fixed rear electrode. The mobile electrode is generally constituted by a deformable membrane covered with a conductive layer. The mobile electrode can also be constituted by a conductive plate, or according to other known configurations.
The mobile electrode and the fixed electrode thus form the armatures of a capacitor, charged by a direct voltage. An acoustic pressure exerted on the mobile electrode causes its displacement with respect to the rear electrode, generally by deformation of the membrane which constitutes it. This leads to a variation in the capacitance formed between the mobile electrode and the fixed rear electrode.
As the electric charge of the capacitor thus constituted is held constant and equal to the product of the voltage and the capacitance, the variation in the capacitance produces an inverse variation in voltage.
In the known state of the art, this type of sensor corresponds to a microphone technology. Microphones are configured so that over their bandwidth they have the most constant sensitivity possible. Their bandwidth extends as widely as possible over a band situated between approximately 20 Hz and approximately 20 kHz, which correspond to the whole of the audible spectrum.
In a capacitive electroacoustic transducer, the coupled system constituted by the mobile electrode, a dissipative element (i.e. capable of causing a dissipation of energy), and typically capable of being an air space situated between the mobile electrode and the fixed electrode, and a cavity, has a natural frequency, which corresponds to a resonance frequency of the electroacoustic transducer in a natural resonance mode. This is the case for any capacitive electroacoustic transducer. In the case of a capacitive electroacoustic transducer the mobile electrode of which is a deformable membrane, the resonance frequency can be defined—to a certain extent—by adjusting the strain of the membrane.
A sensitivity which is the most constant possible over the bandwidth for which the microphone is configured is obtained on the one hand, by configuring the transducer in order to move its first resonance frequency beyond the bandwidth used, and on the other hand by damping this resonance.
Typically, for a microphone, it is standard practice to shift this resonance frequency towards the high frequencies, for example beyond 9 KHz or more depending on the applications: up to 140 kHz for microphones intended for measurements on models (in order to retain a wavelength on the scale of the model, a frequency must consequently be used that is shifted towards the high frequencies), even up to 0.5 MHz for the study of shock waves or animals emitting ultrasound such as for example bats. As regards obtaining a damping which allows the attenuation of the resonance peak in the main mode or in other modes which can be situated within the bandwidth used, the presence of a film of air between the membrane and the electrode induces a damping caused by the viscous losses in air. This acoustic resistance will influence the quality factor, which is a predominant parameter for the characterization of the behaviour of the electroacoustic transducer. The quality factor is a dimensionless parameter which characterizes the damping factor of an oscillating system.
The quality factor can be measured or calculated in various known ways. It is defined as the ratio of the natural frequency, at which the gain is maximum, to the width of the bandwidth of the system at −3 dB of the resonance level.
The higher the quality factor, the lower the bandwidth, and the more the resonance is indicated by a significant gain peak, i.e. high and narrow.
Thus, an electroacoustic transducer must have a low quality factor, indicating the absence of a significant resonance peak.
There are multiple applications for an electroacoustic transducer operating as a sensor. In certain applications, it is advisable to determine whether the electroacoustic transducer is exposed to a given frequency or not.
To this end, it is known to use a selective electronic filter of the frequency considered, or of the range of frequencies considered. Such a filter nevertheless leads to a certain complexity of implementation, requiring a certain computing power (in the case of a digital filter), and requires an electrical supply which also makes implementation of the system more complex. This complexity is detrimental, in particular in the systems implementing electroacoustic transducers which are small in size and/or in large numbers. A large number of electroacoustic transducers used as sensors can in fact be necessary in order to discriminate several frequencies or ranges of frequencies, and/or in order to determine the exposure to certain frequencies in an extended space, which requires the sensors to be dispersed.
A purpose of the present invention is to resolve at least one of the aforementioned drawbacks.